Let's assume that there is a lechatchila to not use metal object that are mekabel tumah (susceptible to tumah) for something that supports the s'chach ("maamid"), which can be avoided by using something that supports something that supports s'chach ("maamid d'maamid"); but if it was used there is no issue b'dieved with using a maamid that is mekabel tumah. See this Halachipedia article for more information.
What about a notched 2x4 board? Many people use long beams across their sukkos to support their schach -- are these mekabel tumah?
Here's a picture: